One of the Romans' major problems during the battle was the lack of a unified military command. When two consular armies joined forces and fought as one unit, as was the case at Cannae, the consuls would exercise command on alternate days. This resulted in confusion about who was ultimately in command of the army.

The Romans learned this bloody lesson at Cannae and made the necessary change to ensure that there was no confusion about who was in command. By 202 BC, at the Battle of Zama, the Roman commander Scipio Africanus was the general-in-chief of the Roman armies in Africa. This unified command was one of the keys to Scipio Africanus defeating Hannibal and has become a cornerstone principle of Western military doctrine. Over two thousand years later, the United States has failed to recognize the importance of the concept.

In September 2014, President Barack Obama appointed former Marine Gen. John Allen as special presidential envoy for the Global Coalition to Counter the Islamic State of Iraq and Syria. Allen, as an appointee of the State Department, was charged with building and sustaining a coalition “in order to degrade and ultimately destroy” ISIS.

The destruction of a military force is not a mission customarily assigned to an envoy from the State Department. The Department of Defense is the US department equipped, trained, and purpose-built to destroy enemy military forces.

“What is CENTCOM, chopped liver?” retired Marine Gen. Anthony Zinni said in an interview with the Tampa Tribune, in response to the appointment. US Central Command is the unified combatant command responsible for the Middle East.

Allen’s appointment has not been greeted with enthusiasm by all. It is no wonder, since the military forces that are to be used to degrade and destroy ISIS already have a commander, Army Gen. Lloyd Austin, CENTCOM’s combatant commander.

Combatant commanders are organized and trained to serve as the vital link between those who determine national security policy and objectives, and the forces who conduct military operations to achieve those objectives.

Austin has already taken the necessary steps to ensure the fight against ISIS is executed and coordinated by subordinate commands. In early 2013, even before ISIS was dominating international headlines, Austin established a dedicated presence in the region, CENTCOM Forward – Jordan.

Reuters Gen. Lloyd Austin greeting the final convoy of US troops to leave Iraq in 2011.

Once ISIS rose to prominence and the international effort to defeat it became a named operation, CENTCOM established another subordinate command, Combined Joint Task Force – Operation Inherent Resolve, headed by a combat-experienced Army three-star, Lt. Gen. James Terry. Both of these commands are clearly defined subordinate commands of CENTCOM. There is no question about the chain of command.

Statutorily, combatant commanders are “responsible to the President and to the Secretary of Defense for the performance of missions assigned to that command.” The chain of command runs from the president to the secretary of defense to the combatant commander. This would place Austin in charge of the mission to “degrade and destroy” ISIS, as ISIS is operating within CENTCOM’s area of responsibility.

Austin is ultimately responsible for accomplishing the mission. But his authority has been compromised by the addition of Allen and his responsibility for overseeing the destruction of ISIS. According to recent reports,

Gen. Lloyd Austin …  took a dim view of Allen’s role. Austin complained to aides that Allen would report directly to the president — bypassing both himself and Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff.

Austin believed that Allen’s appointment would lead to confusion about who was really leading the effort, a senior US officer who serves with Austin told me several days after the appointment.

There is an additional problem with the introduction of Allen into this situation, specifically that he was not confirmed by the Senate. In February 2013, Allen announced his decision to retire from the Marine Corps after a distinguished 38-year career.

Following his retirement, Allen was appointed as the senior advisor to the secretary of defense on Middle East security, and was then subsequently appointed to serve as special presidential envoy for the global anti-ISIS coalition. Neither of these appointments required Senate confirmation. In contrast, Austin was confirmed by the Senate to serve as commander of US Central Command.

The importance of unity of command for military success should be of paramount importance in the current mission to “degrade and destroy” ISIS. Ashton Carter, the likely incoming secretary of defense, should seek to clarify the command relationship in the ongoing fight against ISIS and that the commander of CENTCOM is responsible for all actions in the fight against ISIS.

To allow confusion regarding the command authority will only result in questionable command relationships and will not assist in mission accomplishment.